Can Manipulation of the Constituents of Ruminant Urine Reduce N2O Emissions from Urine Patches?.

Grazing ruminant livestock deposit 75–90% of their nitrogen (N) intake onto pasture as dung and urine, the latter being of major concern for production of the greenhouse gas nitrous oxide (N₂O). With the demand for food increasing, global ruminant numbers are expected to rise, resulting in elevated levels of urine-derived N₂O. Consequently, strategies to mitigate these emissions are required. Urine contains hippuric acid (HA) and benzoic acid (BA), both of which have previously been linked to mitigating urine-derived N₂O emissions in laboratory studies. A significant reduction of up to 65% was achieved when levels of HA or BA in urine were elevated. If applicable in situ, manipulation of ruminant urine composition could prove a successful N₂O mitigation strategy. However, the sole in situ experiment conducted to date was deemed inconclusive due to environmental conditions not conducive to N₂O loss.

The current in situ study assessed the efficacy of elevated HA and BA concentrations in ruminant urine for mitigating urine-derived N₂O under environmental conditions favouring N₂O loss through denitrification.

Results show that urine deposition onto pasture resulted in an immediate increase in N₂O fluxes with the largest daily N₂O fluxes observed on the day of application. Nitrous oxide peak diminished after five weeks. Urine deposition increased cumulative N₂O emissions, however elevation of HA and BA concentrations in urine did not reduce N₂O fluxes. As a consequence it was concluded that HA and BA concentration manipulation are ineffective for reducing N2O loss from in situ applications of urine.

Although simulation of dietary manipulation to reduce N₂O emissions through altering individual urine constituents appeared to have no effect this is an important area and there may be other manipulations such as reducing urine N content or inclusion of synthetic inhibitory products that warrant further investigation.